Method of regenerating citric acid etching solutions by ion exchange



United States Patent 3,116,240 METHOD OF REGENERATING CITRIC ACID ETCHING SOLUTIONS BY ION EXCHANGE Thomas A. Downey, West lslip, and John J. McCallion,

Brooklyn, N.Y., assignors to Chas. Pfizer & (10., Inc,

New York, N.Y., a corporation of Delaware N0 Drawing. Filed Apr. 7, 1961, Ser. No. 103,184

2 Claims. (1. 210-24) This application is concerned with a new and useful process employing cationic ion exchange resins to increase the relative proportion of ferric ion and ferric citrate chelate ion in a citric acid etching solution.

This application is a continuation-in-part of patent application Serial Number 922, filed January 7, 1960, and now abandoned.

There are many instances where citric acid may be successfully utilized to etch a metal surface, that is, a metal surface in which the predominant metal is iron, to prepare the surface for subsequent treatment. There may be mentioned by way of example the prior etching of enamelling iron or equivalent metal with citric acid in a one coat method for applying porcelain. Other applications where citric acid is similarly used, include preparing a metal surface for subsequent plating with another metal, or for applying a plastic coating or a paint. In all of these applications and in others, the surface of the metal is first treated, that is etched, with aqueous citric acid generally by spraying or by dipping to produce microscopic indentations on the surface. These indentations are believed to be associated with the strength of the bond between the metal and the surface coat.

When the metal is etched, there is an accumulation of ferrous and ferric ions in the solution. In some respects, this is advantageous since the ferric ion by reaction with the metallic iron increases the etch rate (see One Coat Porcelain Enamelling Reaches Production at Westinghouse in the December 1960 issue of Metal Products Manufacturing at page 42). Unfortunately, however, most of the ions which form are in the ferrous state and thus do not aid in this respect. Further, with some etching agents, especially citric acid, a precipitate containing ferrous ion forms after substantial quantities of metal have been processed. The precipitate is formed from the citric acid. It thus decreases the quantity of the citric acid in the solution necessitating constant replacement of this reagent.

The precipitate does not interfere with the actual etching so long as sufficient free acid is present. It does, however, present mechanical and scheduling difficulties, especially if irregular objects are to be etched. It may cling to the surface of regular objects or collect in the crevices of irregular objects. In either case, an excessive amount of washing is required to remove it before subsequent treatment. This is an important consideration not only because of the excess water required but also because of the time required for washing. In most industrial procedures requiring etching, the etching is only one step of an assembly line operation in which each step must be carefully synchronized with every other step. If one takes too long, the others may be seriously affected. Thus, if the etched surface requires extra time for washing, the timing for each subsequent step may be affected. This is an especially diflicult problem in citric acid etching because the amount of precipitate in the bath is not constant and continually increases with increased use of the bath. Hence, the amount of washing required varies and is continually changing with the use of the bath. It will be apparent that the washing away of the precipitate has an undesirable result in that bound citric acid is wasted. Obviously, a procedure that makes it possible to prevent formation of the undesirable precipitate and at the same 3,115,240 Patented Dec. 31, 1963 time, keeps ferric ion soluble and available for etching is an important advance in the art. Such a process has now been developed.

In the process of this invention the solution to be treated is contacted with a cationic ion exchange resin which selectively removes ferrous ions and increases the relative proportion of ferric ions in the solution.

It has now been discovered that citric acid is unique among the useful etching acids by reason of the fact that it forms a strong soluble chelate with ferric ions. The chelate has an ionic charge of plus one. The intramolecular forces are such that the chelate is not decomposed by contact with a cationic ion exchange resin. The chelate with ferrous ion, if indeed one forms, is decomposed by contact with the cationic ion exchange resin. Thus, if one contacts a solution containing a mixture of ferric ion, ferric citrate chelate ion, citric acid and ferrous ion, which may be chelated, with a cationic ion exchange resin, the ferrous ion will be adsorbed and there will be in the resulting solution an increase in the relative proportion of ferric ion. This ferric ion whether in the form of free ion or in the chelate form is then available to enhance the etch rate of the solution when it is again contacted with the metal to be etched. It will be apparent also that the solution is at the same time enriched with respect to citric acid by a combination of two factors. First, the citric acid originally bound to the ferrous ion is set free when this ion reacts with the resin. Second, the citric acid originally chelated with ferric ion is regenerated when the chelate decomposes to form an equilibrium mixture of ferric ion and citric acid.

If citric acid did not form the unexpectedly strong chelate with ferric ion the effect of contacting a mixture of ferric and ferrous ions with a cationic ionexchange resin would be the reverse of the above. The ferric ion would be adsorbed in preference to the ferrous ion. The reason for this is that the ferric ion has an ionic charge of plus three while the ferrous ion has an ionic charge of only plus two as stated above. The ferric ion chelate has a charge of only plus one. Hence, divalent ferrous ion is selectively adsorbed in preference to the chelate- The original mixture which is contacted with a cationic ion exchange resin will contain ferric ion in equilibrium with citric acid and ferric citrate chelate ion. The ferric ion will be absorbed in preference to any other ion present in the solution. However, any ferric ion which is adsorbed on the resin is desorbed on continued contact of the resin with citric acid. This is because the molecular forces which attract the ferric ions to the resin are not as strong as the molecular forces tending to form the chelate between the ferric ion and citric acid. Hence, the efiluent from an ion exchange column will contain ferric ion chelate. This will decompose to some extent to form an equilibrium between ferric ion, ferric ion chelate and citric acid.

In one application of process of this invention, a 10% citric acid solution containing 29 g. per gallon of total iron consisting of 23 g. per gallon of ferrous ion and 6 g. per gallon of ferric ion was contacted with a cationic ion exchange resin calculated to remove 23 g. of total iron. It was found that the solution after removal of the resin contained 6 g. of ferric ion mostly in the chelate form and no detectable ferrous ion.

One commercial application of this invention is to the porcelainizing process described and claimed in co-pending application, Serial Number 768,081, filed October 10, 1958. This application refers to a process for the porcelain enamelling of ferrous metal objects comprising the steps of cleaning the ferrous metal, pickling with an aqueous solution of citric acid containing from about 5% to about 20% by weight of acid at a temperature of from about to about 212 F. for from about 5 to about 30 minutes, depositing nickel on the surface in an amount of from about 0.03 to about 0.15 g. per square foot by immersion in an aqueous nickel sulfate bath containing from about l/z ounce to about 3 ounces of nickel sulfate per gallon of water for a period of from about 4 to about 8 minutes at from about 160 to about 170 F. at ,a pH of from about 2.5 to about 3.6, treating the object with nickel deposited on the surface thereof with porcelain enamel, drying at from about 250 to about 400 F. and baking it from about '1300 to about 1500 F. for from about 3 to about 6 minutes.

In a typical operation of the process, a threeainch by six-inch piece of 12 gauge enamelling iron suitable for the preparation of porcelain enamelled tile is cleaned by immersion in aqueous sodium hydroxide and rinsed according to standard practice. It is then immersed in a by weight aqueous solution of citric acid at 190 F. for 30 minutes. It is removed and rinsed with water at room temperature. The article is then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of 2 ounces per gallon for a period of 4 minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid citric acid. The temperature is maintained between 160 and 170 F. This treatment deposits approximately 0.05 to 0.07 gram of nickel per square foot of surface. The iron is then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of one-half ounce per gallon. The temperature is maintained at approximately 185 F. for 2 minutes. The article is then rinsed with water at room temperature and dried at about 250 F. The article is then baked at 1300 F. for 6 minutes.

The said copending patent application points out the advantages of the process claimed therein compared to the prior art processes. The latter all require two coats of porcelain, or if only one coat is required, are economically unattractive because they require specially prepared metals or for some other reason. The process claimed therein is a most important advance in the art and permits the preparation of porcelainized objects with bond and appearance qualities equal to or superior to those heretofore obtained with substantial saving in time and money.

As stated above, the process suffers the of forming a precipitate after substantial quantities of combined with ferrous ion.

It has been found that with a two-thousand gallon capacity aqueous citric acid bath, precipitate will form when approximately sixty-eight thousand square feet of surface to 34 square feet of surface per gallon of aqueous citric acid. During the processing, the concentration of citric acid in the solution will decrease and may be maintained within operable limits, that is, 5% to 20% by the addition of solid citric acid. The concentration of citric acid, however, is immaterial with respect to the formation of a precipitate. The important factors are the amount of water present and the square feet of surface processed. The relationship is approximately linear. Thus, if 1,000 gallons of solution are used, precipitate will form after approximately 34,000 square feet of surface have been processed and with 4,000 gallons of solution, approximately 136,000 square feet of surface may be processed. It is apparent then, that the precipitant builds up in the solution until the solution is saturated or supersaturated with precipitant and the latter then forms a precipitate.

Precipitation may be prevented by removing the dissolved material. This is accomplished by contacting the solution with cationic ion exchange resin. The resin treatment takes place at some time before the calculated point at which precipitation will take place, that is, some time before more than approximately 34 square feet of surface, preferably approximately 30 square feet of surface,

has been processed for each gallon of solution. The actual point at which treatment is carried out is immaterial. It may, in fact, as will be explained more fully below, be a continuous treatment. If it is an intermittent treatment, it is usually most efiicient to pass the maximum amount of surface through the bath which is just insufficient to cause precipitate formation and to then contact the solution with the resin.

Contact may be by any of the usual means. Thus, one may simply add the resin to the solution and stir at room temperature. The duration of stirring is not critical and depends only on the amount of the solution to be treated. At the end of the contact period, the resin is removed, conveniently by filtration and the solution is ready for reuse. It may be desirable to add a small amount of citric acid to the regenerated bath to bring the concentration up to the desired level.

In another method of contact, the solution is passed through a column containing the resin. The effluent from the column which contains dissolved citric acid and forric ion chelate may be directly used as a regenerated bath.

In the continuous method of operation, the tank containing the solution is connected in series with a pump and a cationic ion exchange bed. In practice, portions of verted for some other use. operation makes it possible to process an unlimited amount of surface without replenishing the citric acid since the bound acid is released by the ion exchange treatment and returned to the bath. In practice, however, small amounts of citric acid are lost by leakage, drag out, resin regeneration or other mechanical means.

Although the foregoing discussion primarily illustrates t e process of the invention as it applies to a one coat procedure for porcelainizing ferrous The preferred concentration of citric acid for porcelainizing ferrous metal objects is from about 5% to about 20% by weight. This may vary somewhat if the metal is to be etched for another use, for example, electroplating or deposition of a plastic coating. The problem of the undesirable precipitate of valuble materials will remain.

In some applications of the process of this invention, mixtures containing acids other than citric acid may be used, Thus, for example, in the preparation of a metal for porcelainizing, etching may be effected with a mixture of citric and glycolic acid.

styrene and divinylbenzene. This type of resin is described in US. Patents 2,191,853; 2,366,077; 2,518,420 and in BIOS 621, No. 22 (1446). They are commercially available. Weaker cationic resins of the carboxylic acid type can be used, but less satisfactorily since they do not completely remove the precipitant. These resins ents 2,319,359; 2,333,754; 2,340,110; 2,340,111 and in BIOS Report 621, No. 22 (1446). They are commercially available. Commercially available embodiments of sul onated polystyrene type resins include Amberlite IR- and Dowex-50 or 50W. Commercially available carboxylic acid resins include IRC-SO and XE-89.

It should be pointed out that citric acid which is not bound to ferric ion will not be adsorbed by the resin. It

will stay in the solution along with the ferric ion chelate and may, of course, be reused for etching.

The essence of this invention is the discovery that it is possible to increase the relative proportion of ferric ion as free ferric ion or as ferric ion chelate in a citric acid etching solution containing dissolved ferric ion, ferric citrate chelate ion and ferrous ion by contacting the solution with a cationic ion exchange resin to remove ferrous ion. The actual amount of total iron in the solution is immaterial since the process can be operated in a continuous manner or intermittently. If operated continuously, the solution may be contacted with the cationic ion exchange resin at such a rate as to maintain the total iron in the solution at any desired level. If operated intermittently, the solution is contacted with the cationic ion exchange resin at any selected point. Usually the selected point will be at some concentration of total iron insufficient to cause precipitation.

An important advantage of the process is that it prevents the formation of a mechanically interfering precipitate containing bound citric acid.

The following examples are given by way of illustration only and are not to be construed as limitations of this invention, many apparent variations of which are possible without departing from the spirit or scope thereof.

Example I A one square foot of enamelling iron suitable for the preparation of porcelain enamelled tile was cleaned by immersion in aqieous sodium hydroxide and rinsed according to standard practice. It was then immersed in one gallon of a 10% by weight aqueous solution of citric acid at 190 F. for 10 minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of 2 ounces per gallon for a period of 4 minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid citric acid. The temperature was maintained between 160 and 170 F. This treatment deposited approximately 0.05 to 0.07 gram of nickel per square foot of surface. The iron was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of one-half ounce per gallon. The temperaturewas maintained at approximately 185 F. for 2 minutes. The article was then rinsed with water at room temperature and dried. The iron was then coated by spraying with commercial porcelain enamel and dried at about 250 F. The article was then baked at 1300 F. for 6 minutes.

It was found that a precipitate forms after 17 of these pieces had been processed through the citric acid bath. This amounts to 34 square feet of surface.

In a concurrent operation, processing was after the 15th piece and interrupted 1000 grams of Dowex-SO was added to the citric acid bath. The mixture was stirred for 15 minutes and filtered to remove the resin. The regenerated bath was then used to process 16 additional pieces before precipitate started to form.

Example 11 Steel plating having dimensions of approximately 4 feet by 8 feet, suitable for use in road signs was cleaned by allowing the vapors of ethylene dichloride to condense on its surface. It was then immersed in a 2000 gallon tank containing 20% by weight aqueous citric acid at 200 F. for 5 minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of one-half ounce per gallon for a period of 8 minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid boric acid. The temperature was maintained between 160 and 170 F. The iron was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of one-half ounce per gallon. The temperature was maintained at approximately F. for 15 minutes. The article was then rinsed with water at F. and dried. The iron was then coated by immersion with commercial porcelain enamel and dried at about 400 F. The article was then baked at 1500 F. for 3 minutes.

It was found that after approximately 68,000 square feet of surface had been processed, a precipitate started to form.

In a concurrent operation, the citric acid bath was connected in series with a pump and a resin column containing 200 cubic feet of Dowex-50W and the solution was continuously passed through the column at a rate of approximately 2 gallons per cubic foot of resin per minute. No precipitate formed in this operation even though the bath was in continuous use for more than a month. It was only necessary to occasionally add small amounts of solid citric acid to the etching bath to maintain the concentration of the bath at approximately 20%. The resin was occasionally regenerated using 10% aqueous sulfuric acid.

*xample Ill Pieces of 12 gauge enamelling iron suitable for the preparation of porcelain enamelled tile were cleaned by immersion in aqueous sodium hydroxide and rinsed according to standard practice. The articles were then immersed in 1000 gallons of a 15% by weight aqueous solution of citric acid at 180 F. for 15 minutes. They were removed and rinsed with water at room temperature. The articles were then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of 2 ounces per gallon for a period of four minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid citric acid. The temperature was maintained between 160 and 170 F. This treatment deposited approximately 0.05 to 0.07 gram of nickel per square foot of surface. The iron was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of one-half ounce per gallon. The temperature was maintained at approximately 212 F. for 2 minutes. The articles were then rinsed with water at 212 F. and dried. The iron was then coated by spraying with commercial porcelain and dried at about 250 The articles were then baked at 1300 F. for 6 minutes.

It was found that after approximately 33,000 square feet of surface had been processed, the precipitate started to form.

In a concurrent operation, an identical citric acid bath was used and the processing was interrupted after 15,000 square feet of surface had been treated. One-hundred and fifty cubic feet of Amberlite 111-120 was added and the mixture stirred at room temperature for one-half hour. The resin was removed by filtration and the regenerated bath returned to use.

Example IV In an operation similar to that described in Example III, the citric acid bath instead of being treated by the addition of Amberlite lit-120, was passed over a column of Amberlite IR-120 in a suitable apparatus at a rate of 2 gallons per cubic foot of resin per minute. Suliicient citric acid was added to the efliuent to bring the concentration back to 15%.

Example V A one square foot piece of enamelling iron suitable for the preparation of porcelain enamelled tile was cleaned by immersion in aqueous sodium hydroxide and rinsed according to standard practice. It was then immersed in one gallon of a 10% by weight aqueous solution of citric acid at F. for 10 minutes. it was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of 2 ounces per gallon for a period of 4 minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid citric acid. The temperature was maintained between 160 and 170 F. This treatment deposited approximately 0.05 to 0.07 gram of nickel per square foot of surface. The iron was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of onehalf ounce per gallon. The temperature was maintained at approximately 185 F. for 2 minutes. The article was then rinsed with water at room temperature and dried. The iron was then coated by spraying with commercial porcelain enamel and dried at about 250 F. The article was then baked at 1300 F. for 6 minutes.

It was found that a precipitate forms after 17 of these pieces had been processed through the citric acid bath.

In a concurrent operation, processing was interrupted after the 15th piece and 1000 grams of IRC50 was added to the citric acid bath. The mixture was stirred for 15 minutes and filtered to remove the resin. The regenerated bath was then used to process 15 additional pieces before precipitate started to for Example VI A mixture of methacrylic acid with divinylbenzene is polymerized by adding 1% benzoyl peroxide catalyst and heating in a closed vessel at 60 C. for 24 hours. The resulting polymer is ground to fine particle size, washed with 8% aqueous sodium hydroxide, rinsed with Water and screened to a mesh range of about 14 to 45 on the US. standard sieve scale.

The screened resin is treated with 3 volumes of 2 N hydrochloric acid and water-washed.

A cation exchange resin is prepared in the following manner: a mixture of inhibitor-free styrene together with 1% dlvinyl benzene and 0.5% benzoyl peroxide is added to twice its volume of water in a reaction vessel equipped with agitator and reflux condenser. A trace of magnesium carbonate is added to facilitate suspension and the mixture is vigorously agitated and heated at reflux until polymerization is complete. The resulting bead polymer is separated, washed and sulfonated by heating at 100 C. with excess concentrated sulfuric acid in the presence of silver sulfate for hours. Washing, drying, and screening yield a resin having a particle size ranging from about 125 to 200 mesh, a moisture content of about 5% and a total exchange capacity of about 5 mic. per gram.

These resins were used in the following examples.

A one square foot piece of enamelling iron suitable for the preparation of porcelain enamelled tile was cleaned by immersion in aqueous sodium hydroxide and rinsed according to standard practice. It was then immersed in one gallon of a 10% by weight aqueous solution of citric acid at 190 F. for 10 minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of one-half ounce per gallon for a period of 8 minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid boric acid. The temperature was maintained between 160 and 170 F. The iron was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of one-half ounce per gallon. The temperature was maintained at approximately 165 F. for 15 minutes. The article was then rinsed with water at 185 F. and dried. The article was then rinsed with water at room temperature and dried. The iron was then coated by spraying with commercial porcelain enamel and dried at about 250 F. The article was then baked at 1300 F. for 6 minutes.

It was found that a precipitate forms after 17 of these pieces had been processed through the citric acid bath. In a concurrent operation, processing was interrupted after the 15th piece and 1000 grams of carboxylic acid resin was added to the citric acid bath. The mixture was stirred for 15 minutes and filtered to remove the resin.

The regenerated bath was then used to process 15 additional pieces before precipitate started to form.

When a sulfonated polystyrene resin was used in an identical manner, 17 additional pieces could be pickled before precipitate started to form.

Example VII A one square foot of enamelling iron suitable for the prepartion of porcelain enamelled tile was cleaned by immersion in aqueous sodium hydroxide and rinsed according to standard practice. It was then immersed in one gallon of a 10% by weight aqueous solution of citric acid at 190 F. for 10 minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of 2 ounces per gallon for a period of 4 minutes while maintaining the pH between 3.2 and and 3.6 by the occasional addition of solid citric acid. The temperature was maintained between and F This treatment deposited of nickel per square foot immesed in an aqueous approximately 0.05 to 0.07 gram of surface. The iron was then solution containing borax and soda ash in a ratio of 2:1 at a concentration of one-half ounce per gallon. The temperature was maintained at approximately F. for 2 minutes. The article was then rinsed with water at room temperature and dried. The iron was then coated by spraying with commercial porcelain enamel and dried at about 250 F The article was then baked at 1300 F. for 6 minutes.

It was found that a precipitate forms after 17 of these pieces had been processed through the citric acid bath. This amounts to 34 square feet of surface.

In a concurrent operation, processing was interrupted after the 15th piece and 1000 grams of the sulfonated resin prepared in accordance with Example VI was added to the citric acid bath. The mixture was stirred for 15 minutes and filtered to remove the resin. The regenerated bath was then used to process 16 additional pieces be fore precipitate started to form.

Example VIII Steel plating having dimensions of approximately 4 feet by 8 feet, suitable for use in road signs was cleaned by allowing the vapors of ethylene dichloride to condense on its surface. It was then immersed in a 2000 gallon tank containing 20% by weight aqueous citric acid at 200 F. for 5 minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel tration of one-half ounce per gallon for a period of 8 minhalf ounce per gallon. The temperature was maintained F. and dried. The iron was then coated by immersion with commercial porcelain enamel and dried at about 400 F. The article was then baked at 1550 F. for 3 minutes.

It was found that after approximately 68,000 square feet of surface had been processed, a precipitate started to form.

Example VI and the solution was continuously passed through the column at a rate of approximately 2 gallons per cubic foot of resin per minute. No precipitate formed in this operation even though the bath was in continuous use for more than a month. It was only necessary to occasionally add small amounts of solid citric acid to Example IX Pieces of 12 gauge enamelling iron suitable for the preparation of porcelain enamelled tile were cleaned by immersion in aqueous sodium hydroxide and rinsed according to standard practice. The articles were then immersed in 1000 gallons of a by weight aqueous solution of citric acid at 180 F. for 15 minutes. They Were removed and rinsed with water at room temperature. The articles were then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of 2 ounces per gallon for a period of four minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid citric acid. The temperature was maintained between 160 and 170 F. This treatment deposited approximately 0.05 to 0.07 gram of nickel per square foot of surface. The iron was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of one-half ounce per gallon. The temperature was maintained at approximately 212 F. for 2 minutes. The articles were then rinsed with water at 212 F. and dried. The iron was then coated by spraying with commercial porcelain and dried at about 250 F. The articles were then baked at 1300 F. for 6 minutes.

It was found that after approximately 33,000 square feet of surface had been processed, the precipitate started to form.

In a concurrent operation, an identical citric acid bath was used and the processing was interrupted after 15,000 square feet of surface had been treated. One hundred and fifty cubic feet of the carboxylic resin of Example VI was added and the mixture stirred at room temperature for one-half hour The resin was removed by filtration and the regenerated bath returned to use.

Example X In an operation similar to that described in Example IX, the citric acid bath instead of being treated by the addition of the carboxylic resin of Example VI, was passed over a column of the carboxylic resin of Example VI in a suitable apparatus at a rate of 2 gallons per cubic foot of resin per minute. Sufficient citric acid was added to the efiiuent to bring the concentration back to 15 Example XI A citric acid etching solution containing 42.5 g. per gallon of total iron consisting of 35 g. per gallon of ferrous ion and 7.5 g. per gallon of ferric ion was contacted with sufficient Amberlite IR- to adsorb 35 g. of iron. The effluent was analyzed and no detectable amount of ferrous ion was found. The effluent was used to each one square foot piece of iron and it was found that the etching rate of the resin contacted solution was greater than the etching rate of the original solution. The etched iron article was then used in experiments in which other metals are plated on the surface in accordance with standard procedures and in still other experiments in which various plastics and paints were deposited on the surface. In all instances, it was found that bond and appearance were of acceptable quality.

What is claimed is:

1. A process which comprises contacting a citric acid etching solution containing dissolved ferrous ion, ferric citrate chelate ion and ferric ion with a cationic ion exchange resin to remove ferrous ion and increase the relative proportion of ferric ion and ferric citrate chelate ion in the solution.

2. A process as in claim 1 wherein the etching solution is contacted with the cationic ion exchange resin before more than about 34 square feet of surface has been etched for each gallon of solution.

Ledford et al.: Ind. and Eng. Chem., vol. 47, No. 1, January 1955, pages 83-86.

Fradkin et al.: Ind. and Eng. Chem., vol. 47, No. 1, January 1955, pages 87-90. (Copy in Scientific Lib.) 

1. A PROCESS WHICH COMPRISES CONTACTING A CITRIC ACID ETCHING SOLUTION CONTAINNG DISSOLVED FERROUS ION, FERRIC CITRATE CHELATE ION AND FERIC ION WITH A CATIONIC ION EXCHANGE RESIN TO REMOVE FERROUS ION AND INCREASE THE RELATIVE PROPORTION OF FERRIC ION AND FERRIC CITRIC CHELATE ION IN THE SOLUTION. 